A CMOS image sensor, which mounts a general column analog-to-digital conversion circuit, includes a pixel array in which a plurality of pixel cells are laid out two-dimensionally, a column analog-to-digital conversion circuit for converting pixel signals into digital signals, and a row selection circuit. Each pixel includes a photoelectric conversion element (for example, photodiode) for converting an optical signal from an object into an electrical signal (charge), and a plurality of field effect transistors.
In the operation of the image sensor, the row selection circuit selects pixels from which signals are to be read for each row. Then, pixel signals are simultaneously output from a plurality of selected pixels on the selected row onto signal lines. The output pixel signals are converted into digital signals by the column analog-to-digital conversion circuit.
In the selected pixel, respective transistors are driven to transfer a charge accumulated in the photoelectric conversion element to a signal detection unit, thus detecting and amplifying a charge amount which is accumulated in the photoelectric conversion element and is transferred to the signal detection unit.
In order to improve image quality and reliability of the image sensor, not only the operations of the row selection circuit and analog-to-digital conversion circuit but also the driving method and control method of the transistors in each pixel have been examined.